On DHCP events and changes to the active configuration the process vyos-hostsd is responsible to issue a command to PowerDNS via rec_control in order to get the forwarding resolver to load the new changes.
Since T4652 however, the upgrade to PDNS 4.7 has changed the name of the process that PDNS runs under, causing vyos-hostsd to believe PDNS is not running even when it is, which then inhibits the calls to rec_control. The end effect being PDNS never getting any state updates from it's configuration files or zone files unless it's manually restarted or a manual rec_control with the appropriate arguments is issued.
Evidence of this issue can be seen by running journalctl -eu vyos-hostsd after an apply operation takes place, displaying:
vyos-hostsd: pdns_recursor not running, not sending "reload-lua-config"
vyos-hostsd: pdns_recursor not running, not sending "reload-zones"